FIG. 7 of the accompanying drawings schematically illustrates a conventional butterfly valve structure 10 in which a butterfly plate 12 positioned in a duct 14 is rotated about an axis 16 defined by a spindle or shaft 18 in order to vary the rate at which fluid flows through the duct. Typically, the plate 12 is rotated via torque applied by an external actuator 20 through the shaft 18.
A series of arrows 22 generally illustrates the static pressure profile resulting from the hydrodynamic forces acting on the butterfly plate 12. The overall effect of the static pressure may be represented by a center of pressure (indicated by the dashed arrow 24) which tends to force the plate 12 to the closed position. This force must be countered or overcome by energy supplied to the actuator 20 in order to retain the position of the plate 12 or further open the valve 10.
As a general matter, in order to lower the externally-supplied energy required to match or exceed the hydrodynamic forces acting on the plate 12, the center of pressure 24 should be favorably altered in relation to the axis of rotation 16. That is, either the center of pressure 24 should be aligned with or moved closer to the shaft 18, or the shaft should be aligned with or moved closer to the center of pressure. Given a particular rotational position of the plate 12 within the duct 14, the center of pressure 24 can be moved in the manner disclosed in U.S. Pat. No. 3,971,414 Illing, for example, by angling a portion of the plate 12 to partially compensate for excessive hydrodynamic forces otherwise acting on the plate at that position. However, this partial compensation is achieved at the expense of splitting the plate 12. This creates problems in applications which demand sealing engagement of the plate 12 with the duct 14 when the valve 10 is closed, and still requires the use of an external actuator 20 to torque the shaft 18.
Butterfly valve structures 10 are in some environments subjected to considerable vibration which can adversely effect both performance and longevity. Vibrational effects are aggravated to the degree that the center of mass of the structure 10 is distanced from the center of the duct 14. Such distancing is the typical state of affairs when a single external actuator 20 is used to torque the shaft 18.
If a butterfly valve structure 10 were designed such that the plate 12 could be translated (rather than simply rotated) relative to the shaft 18, then it would be possible to align or more closely align the center of pressure 24 with the axis 16, thus eliminating or minimizing the externally-supplied energy required to maintain a given flow rate. Furthermore, if the center of mass of the actuator 20 more closely coincided with the center of the duct 14, then the forementioned vibrational effects would be minimized. Accordingly, the present invention is directed to the objective of providing a butterfly valve that is more energy-efficient than conventional butterfly valves. An additional objective of the invention is to provide a structurally stable butterfly valve that is less sensitive to vibration than conventional butterfly valves.